Locating multiple non-patterned holes on an irregularly shaped flat surface 1

[dude_inthe_NB]

I have a cast part that is not square, round, or any normal profile shape. It has 3 mounting holes and must bolt to another part that I do not have access to tolerances for. (aftermarket part). I am trying to control the location of the mounting 3 holes with each other so that the part will always mount onto the OEM part. With a more normally shaped part, I could use the mating surface as datum A and the sides as datums B and C. Problem is the sides aren't square and their orientations don't drive the fitment of the part. I have the mounting surface set as datum A with 1 hole set as datum B and perpendicular to datum A. I am struggling with how to properly locate the other 2 holes as all 3 located within tolerance to assembled. Which hole should be datum C to constrain the part and what should the position tolerance frame look like for the other 2 holes? and how should the dimensioning scheme be done since all 3 holes are equally important?

Every example I can find on the internet is with a nice square or round part and uses the sides of the part as datums B and C. Obviously I cannot apply the same logic to my part. What is the proper way to control these holes?

See the attachment for a visual representation of what I am trying to describe.

 

[3DDave]

You could use all of the mounting holes as a pattern and identify them as secondary a datum feature with the mounting face as primary. See Fig. 4-26 Hole Pattern Identified as Datum in the ASME Y14.5-2009 version for an application like this. Similar figure 4-22 in the 1994 version.

 

[chez311]

The sides not being flat isn't a barrier to use as a datum feature (thats where datum targets and complex profile datum features come into play) but if they don't drive fitment or function then for sure one should try and choose features which do.

Assuming no one hole is more "important" or drives location/orientation of the assembly more than the other(s) then you can set the entire 3X pattern as a datum feature.

The more elegant solution, but sometimes less palatable, is to set the 3X holes with position to A and specify your other tolerances (ie: profile of the outside surface) to A. That way simultaneous requirements holds your tolerance zones in common location/orientation.

 

[dude_inthe_NB]

With this approach, what dimensions are required to locate the holes to each other? Just 4 basic dimensions (X & Y from the first hole to the other 2)?

 

[TheTick]

Make the holes the primary datums and dimension/tolerance/control the rest from there.